Cessna 172 violent shimming.

B

bluesideup

Line Up and Wait
Joined
Jun 20, 2014
Messages
672
Display Name
Display name:
bluesideup
Hello everyone.

C172 started to shimmy / shake under normal landing / light load on the front wheel.

Everything seems to be tight and no visible leaks anywhere. The dampener also looks good.

Any ideas where to look for the not so obvious, what the problem may be?
 
bluesideup said:
Hello everyone.

C172 started to shimmy / shake under normal landing / light load on the front wheel.

Everything seems to be tight and no visible leaks anywhere. The dampener also looks good.

Any ideas where to look for the not so obvious, what the problem may be?
Click to expand...

Tie the tail down to get nose wheel off the ground. From the side, firmly grab the tire and try to move or 'steer' the wheel and look for lost motion / backlash. A short towbar may make this easier, but nose wheel must be off ground. You may as well remove the nose wheel fairing, until you identify and fix the problem because a violent shimmy is going destroy it anyhow.

Violent shimmy can be caused by a number of things, such as:

NOSE WHEEL BALANCE - While the nose wheel is in the air, rotate it and see if it settles to a heavy spot.

TIRE SEVERELY OUT-OF-ROUND - probably from an out of balance wheel touching down in the same (heavy) spot

SHIMMY DAMPER INEFFECTIVE - Remove it and fill with 5606 hydraulic oil

NOSE WHEEL BEARINGS - Little to no excess play - Not too tight either - Don't forget to reinstall cotter pins on shaft

LOOSE TORQUE LINKS - Lost motion in steering direction

STEERING ARM COLLAR SHIMS - Lost motion seen as an UP/DOWN wobble at the steering arms.

Steering collar backlash is often overlooked, mainly because it is a P.I.T.A.

If you see a up/down wobble at the steering arms, use a feeler gauge to check for collar bearing end play. Cessna steering collar shims come in three thicknesses - .006 / .012 / .020. If you can easily fit a .006 feeler in there, then nose strut must be removed to add or replace shims.

If you can locate a copy of CPA Tech Note 001, it goes into all the above in great detail.

Hope this helps,
Jerry King
 
Hi.
Thank you for the help.
I will have look, some of the suggestions I did not think about.
 
eman1200 said:
hey Tom, what's an "Axel" ?



(I just wanted to steal @GlennAB1 's thunder plus it's been a while since I ruffled up a thread)
Click to expand...

Every one already knows I can't spell. and some times spell checker makes it worse.
 
Jerry King said:
Tie the tail down to get nose wheel off the ground. From the side, firmly grab the tire and try to move or 'steer' the wheel and look for lost motion / backlash. A short towbar may make this easier, but nose wheel must be off ground. You may as well remove the nose wheel fairing, until you identify and fix the problem because a violent shimmy is going destroy it anyhow.

Violent shimmy can be caused by a number of things, such as:

NOSE WHEEL BALANCE - While the nose wheel is in the air, rotate it and see if it settles to a heavy spot.

TIRE SEVERELY OUT-OF-ROUND - probably from an out of balance wheel touching down in the same (heavy) spot

SHIMMY DAMPER INEFFECTIVE - Remove it and fill with 5606 hydraulic oil

NOSE WHEEL BEARINGS - Little to no excess play - Not too tight either - Don't forget to reinstall cotter pins on shaft

LOOSE TORQUE LINKS - Lost motion in steering direction

STEERING ARM COLLAR SHIMS - Lost motion seen as an UP/DOWN wobble at the steering arms.

Steering collar backlash is often overlooked, mainly because it is a P.I.T.A.

If you see a up/down wobble at the steering arms, use a feeler gauge to check for collar bearing end play. Cessna steering collar shims come in three thicknesses - .006 / .012 / .020. If you can easily fit a .006 feeler in there, then nose strut must be removed to add or replace shims.

If you can locate a copy of CPA Tech Note 001, it goes into all the above in great detail.

Hope this helps,
Jerry King
Click to expand...

The problem with just raising the nose off the ground is that it doesnt collapse the oleo enough to disengage the centering block on the upper torque link, and it masks any slop at that link bolt as well as slop at the steering collar and shimmy damper connections. You have to let the nitrogen out of the oleo and push it up to diagnose all the stuff properly.

And, as usual, everyone goes after symptoms instead of root causes. Shimmy of a nosewheel has the same root cause as shimmy of the front wheels in your car: dynamic imbalance. Looking for a heavy spot is looking for static imbalance, but static imbalance doesnt cause shimmy. Dynamic does, and checking for it involves a lot more work. The tire shop that installes new tires on your car uses electronic dynamic balancing equipment; if they did a static balance your front end would shake something awful sometimes. When I was a kid that was all most cars got, and the steering wheel was shaking at highway speeds.

Finding a shop with the right sort of balancer is tough. I built my own. It stops shimmy issues dead, even on airplanes with worn nosegear stuff. There are several long threads on this forum about dynamic balance and the mechanics of it. Look for those threads.
 
Dan Thomas said:
And, as usual, everyone goes after symptoms instead of root causes. Shimmy of a nosewheel has the same root cause as shimmy of the front wheels in your car: dynamic imbalance. Looking for a heavy spot is looking for static imbalance, but static imbalance doesnt cause shimmy. Dynamic does,
Click to expand...
When balance is bad enough to cause shimming the pilot will feel the shaking more after they raise the nose, than when the tire is on the ground.
But It never hurts to have the tire balanced
 
Dan Thomas said:
The problem with just raising the nose off the ground is that it doesnt collapse the oleo enough to disengage the centering block on the upper torque link, and it masks any slop at that link bolt as well as slop at the steering collar and shimmy damper connections. You have to let the nitrogen out of the oleo and push it up to diagnose all the stuff properly.

And, as usual, everyone goes after symptoms instead of root causes. Shimmy of a nosewheel has the same root cause as shimmy of the front wheels in your car: dynamic imbalance. Looking for a heavy spot is looking for static imbalance, but static imbalance doesnt cause shimmy. Dynamic does, and checking for it involves a lot more work. The tire shop that installes new tires on your car uses electronic dynamic balancing equipment; if they did a static balance your front end would shake something awful sometimes. When I was a kid that was all most cars got, and the steering wheel was shaking at highway speeds.

Finding a shop with the right sort of balancer is tough. I built my own. It stops shimmy issues dead, even on airplanes with worn nosegear stuff. There are several long threads on this forum about dynamic balance and the mechanics of it. Look for those threads.
Click to expand...
Agree. And Tom's answer is a long shot.
 
eman1200 said:
hey Tom, what's an "Axel" ?



(I just wanted to steal @GlennAB1 's thunder plus it's been a while since I ruffled up a thread)
Click to expand...

maxresdefault.jpg
 
Hi everyone.
Just to make sure that there is no confusion, this only occurs during landing, and only if you try to keep the nose/ wheel under a light load / keep the nose up.
Thanks to every one that contributed in a productive manner.
 
bluesideup said:
Hi everyone.
Just to make sure that there is no confusion, this only occurs during landing, and only if you try to keep the nose/ wheel under a light load / keep the nose up.
Thanks to every one that contributed in a productive manner.
Click to expand...
What I said will fix it.
 
Tom-D said:
When balance is bad enough to cause shimming the pilot will feel the shaking more after they raise the nose, than when the tire is on the ground.
But It never hurts to have the tire balanced
Click to expand...
Raising the nose engages the centering cam, which will mask any shimmy instantly. It stops any nosewheel pivoting, which is all shimmy is.
 
Dan Thomas said:
Raising the nose engages the centering cam, which will mask any shimmy instantly. It stops any nosewheel pivoting, which is all shimmy is.
Click to expand...

Only when every thing is tight, when the axle is loose it will shimmy. and it gets worse when the load is light.
 
Jerry King said:
Tie the tail down to get nose wheel off the ground. From the side, firmly grab the tire and try to move or 'steer' the wheel and look for lost motion / backlash. A short towbar may make this easier, but nose wheel must be off ground. You may as well remove the nose wheel fairing, until you identify and fix the problem because a violent shimmy is going destroy it anyhow.

Violent shimmy can be caused by a number of things, such as:

NOSE WHEEL BALANCE - While the nose wheel is in the air, rotate it and see if it settles to a heavy spot.

TIRE SEVERELY OUT-OF-ROUND - probably from an out of balance wheel touching down in the same (heavy) spot

SHIMMY DAMPER INEFFECTIVE - Remove it and fill with 5606 hydraulic oil

NOSE WHEEL BEARINGS - Little to no excess play - Not too tight either - Don't forget to reinstall cotter pins on shaft

LOOSE TORQUE LINKS - Lost motion in steering direction

STEERING ARM COLLAR SHIMS - Lost motion seen as an UP/DOWN wobble at the steering arms.

Steering collar backlash is often overlooked, mainly because it is a P.I.T.A.

If you see a up/down wobble at the steering arms, use a feeler gauge to check for collar bearing end play. Cessna steering collar shims come in three thicknesses - .006 / .012 / .020. If you can easily fit a .006 feeler in there, then nose strut must be removed to add or replace shims.

If you can locate a copy of CPA Tech Note 001, it goes into all the above in great detail.

Hope this helps,
Jerry King
Click to expand...

Here you go.
http://cessna.org/documents/public_info/sampletech.pdf
My shop made adapters to fit aircraft wheels onto an automotive dynamic balancer. Works great.
 
Maybe it is as simple as an over inflated nose wheel. Have you checked tire pressures?

On a Cirrus, an over inflated nose wheel will cause shimmy to the point of nose wheel collapse.
 
Maybe your tire was low and so you squirted some of this green tire goo into it.
I know a guy who did that once. What a terrible idea. The shimmy on landing was awful.
On takeoff it wasn't noticeable (To the guy I'm talking about) because the goo got distributed during taxi.

But on landing (The guy I know) said it was really bad because the stuff he put in the tire like an idiot pooled during flight and made his tire terribly unbalanced.

Some people... (my friend) SMH...
 
Hello everyone.

I was able to try a few things and we are at the point where a new dampener, heavy duty?, was ordered.

Nose up, the tire, property inflated, was not out of round, at least not that we could observe.

Trying to move the tire / wheel in all directions very little movement on the axle, just barely noticeable. The tire is in very good, almost new, condition, and there was no tire flat fix goop ever used.

The strut and the scissors seem to work fine, if you push the nose down it will come back up to what we think is a normal position about 6 in. from the bottom.

I have a video of one of the landings, by the way it does not do it all / every time, and the motion is more lateral / side to side, not up / down, much like the movement is about the mains, but it only does it after the front / nose wheel touches down.

Thank you, everyone. I will keep you informed.
 
Just say no to nose wheel
 
bluesideup said:
Hello everyone.

I was able to try a few things and we are at the point where a new dampener, heavy duty?, was ordered.

Nose up, the tire, property inflated, was not out of round, at least not that we could observe.

Trying to move the tire / wheel in all directions very little movement on the axle, just barely noticeable. The tire is in very good, almost new, condition, and there was no tire flat fix goop ever used.

The strut and the scissors seem to work fine, if you push the nose down it will come back up to what we think is a normal position about 6 in. from the bottom.

I have a video of one of the landings, by the way it does not do it all / every time, and the motion is more lateral / side to side, not up / down, much like the movement is about the mains, but it only does it after the front / nose wheel touches down.

Thank you, everyone. I will keep you informed.
Click to expand...
Before you buy new, are you certain that your damper is serviced properly?
there is only 1 way to insure that the dampener is totally full. and that is to assembly it when fully submersed in hydraulic fluid
 
James331 said:
Just say no to nose wheel
Click to expand...

Tailwheels have shimmy problems too, but there are some additional factors there that also need addressing. I have not yet seen any tailwheel that doesn't use simple friction to try to control shimmy, which just makes steering a lot harder. So primitive in this day of elastomeric compounds that could be incorporated into a new tailwheel as a damping device without sacrificing steering precision. Lord makes a non-fluid elastomeric shimmy damper now and Cessna has been using it on their newer airplanes. https://www.lord.com/products-and-s...rospace-and-defense/fixed-wing/shimmy-dampers
 
SixPapaCharlie said:
Maybe your tire was low and so you squirted some of this green tire goo into it.
I know a guy who did that once. What a terrible idea. The shimmy on landing was awful.
On takeoff it wasn't noticeable (To the guy I'm talking about) because the goo got distributed during taxi.

But on landing (The guy I know) said it was really bad because the stuff he put in the tire like an idiot pooled during flight and made his tire terribly unbalanced.

Some people... (my friend) SMH...
Click to expand...
I assume we are talking about Carl.

 
Dan Thomas said:
Tailwheels have shimmy problems too, but there are some additional factors there that also need addressing. I have not yet seen any tailwheel that doesn't use simple friction to try to control shimmy, which just makes steering a lot harder. So primitive in this day of elastomeric compounds that could be incorporated into a new tailwheel as a damping device without sacrificing steering precision. Lord makes a non-fluid elastomeric shimmy damper now and Cessna has been using it on their newer airplanes. https://www.lord.com/products-and-s...rospace-and-defense/fixed-wing/shimmy-dampers
Click to expand...

Just seems much less common, especially with a properly rigged scott
 
James331 said:
Just seems much less common, especially with a properly rigged scott
Click to expand...

One some airplanes it's not a problem. Cessna's 180/185 tailspring is the best idea: long, so it doesn't shift the tailwheel's pivot axis too much as it flexes under load, and torsionally stiff so that it doesn't tend to feed any incipient shimmy. Flat tailsprings like a Cub's or Citabrias are shorter and far more twistable, and even a Scott will shimmy if the axis is off or if the wheel is dynamically imbalanced. We had a Citabria on which I often had to replace tailsprings when they flattened out (flight training is hard on them) and the steering axis would tip forward at the top, the same unstable arrangement as a bent shopping-cart caster. One of the Citabrias suffered a broken sternpost due to the violent flexing caused by the shimmy. It's serious stuff.
 
Gave a ton of dual in 7ECAs, only shimmy issue we had was with the one on a Maule Tailwheel

But we were pretty good about installing a good foundation in the students, who where luckily 0 time often, normally low passes to wheel landings (tail low wheels landings) to three point and on from there to cross wind, tailwind, grass, etc.

Only ones who would sometimes prang the tailwheel were the random guy who had a few hours in trikes before he came to us.
 
I would think the newer elastomeric-based shimmy dampers would have damping properties that are incredibly temperature sensitive when compared to more traditional orifice-based hydraulic ones.

I agree w Dan Thomas that linkage slop is the most common form of Cessna shimmy. The trouble is most mechanics don't know how to defeat the auto-centering function of the nose wheel support, yet display the linkage operation and backlash.
 
You might want to check the bolts holding the shimmy dampener. We had bad shimmy in our Cardinal and as I was helping our mechanic rebuild the nose wheel bushings we noticed the two bolts holding the dampener were loose.
 
You must log in or register to reply here.
Back
Top